Infill housing frequently requires the provision of services which cross boundaries and which must be precisely located. When the drainage is one of the services, the fall or incline must be incorporated into the final selected direction. Additionally, where line of sight is available to find the radial angle from the bore entrance to the target site, optical instruments provide accuracy. If an obstruction is encountered, an excavation may be needed to investigate. Alternatively the change in direction is planned. Every effort is made to reduce the expensive boring stage to a minimum. The use of laser technology by drainers is well established, but laser guided micro-tunnelling machines are expensive and not widely used.
U.S. Pat. No. 3,857,449 discloses a pipe thruster which uses a laser beam as a directional reference. The guidance system relies upon detecting the deviation of the machines thrust axis from the optical path of the beam.
Australian Patent No. AU-A-12360/88 describes a guidance control system for a laser guided boring machine for boring underground drains. The laser target has five light sensitive portions which emit voltages which when amplified are compared to predetermined threshold values and an output signal actuates a pair of 24v motors. The motors drive linear actuators which adjust the direction of the boring bit.
Trials and contract boring show that if the electronic components of the device fail, they tend to do so in locations where service and repair is slow or unavailable. It has also been found that when the strata are uniform, surprisingly infrequent corrections are required in practice, but this was only discovered when a non-automatic version was constructed and tested.